In its simplest form a re-breather is a device that enables a person to retain and reuse some or all of his or her expired breath. Even with physical exertion, a person uses only a fraction of the oxygen that is inhaled. A re-breather re-circulates unused oxygen in the system and replenishes the oxygen as it is used by the wearer. This allows a very small tank of oxygen to last much longer than is possible using traditional SCUBA (Self Contained Underwater Breathing Apparatus) gear. The three main components of typical re-breather systems are gas supply/oxygen control, counterlung, and carbon dioxide removal system.
A rebreather has a carbon dioxide removal system that maintains CO2 pressures at a safe level. This is relatively easy to do, and is accomplished by passing exhaled gases through a canister filled with a chemical adsorbent, such as soda lime. Several manufacturers make these adsorbents and use their own special mixes. For example, SODASORB®, manufactured by W. R. Grace & Co., consists of a mixture of sodium hydroxide, calcium hydroxide and potassium hydroxide. Other adsorbents, such as lithium hydroxide, can be used to offer improved cold water performance.
Adsorbents are typically in the form of small granules 0.04 to 0.25 inches (1.0 to 6.5 mm) in diameter, placed in a canister through which exhaled gases are passed. Smaller granules allow more surface area per unit weight, but because the person must “breathe” through this canister without too much resistance, larger adsorbent particles are employed so as to allow gas flow around these granules, and through the canister with a relatively low pressure drop.
Current re-breathers employ canisters that are generally cylindrical in shape and the expelled gas enters the canister through one end of the cylinder and exits at the opposite end. In the past, the design of CO2 removal canisters has been limited to one of three general configurations decided at the time of manufacture. In the Axial Configuration, expelled gases pass directly from one end of the canister to the other through the adsorbent material. In one radial configuration, the expelled gas flows from a tube in the center of the canister and radiates outward through the adsorbent material. Another radial configuration comprises of expelled gases entering the canister and flowing from the outside of the adsorbent material and radiating inward through the adsorbent material.
Because the larger particles of adsorbent material cannot be packed into the canister as densely as the smaller particles, if one desires to increase the available CO2 adsorption capacity of a re-breather one must add additional adsorbents. To increase the volume of adsorption material, one must increase the volume of the canister. In the case of canisters having the radial configuration, any increase in canister volume would be due to an increase in the diameter of the cylinder and/or the length of the cylinder. Any significant increase in cylinder volume could potentially result in a cylinder having a diameter and/or length that would be overly cumbersome and impractical for diving because while a diver was able to stay down longer, he or she would not be able to work as efficiently due to the canister size and/or placement of the canister on the diver's body.